This invention relates to a conductive grease, and in particular to such a conductive grease less to reduce conductivity as time passes.
The invention further relates to a conductive grease having excellent conductivity and being less to reduce the conductivity as time passes, and specially to such a conductive grease to be suitably applied to various kinds of rolling bearings, decreasing electrical resistance between inner and outer races.
The invention still further relates to a rolling bearing such as a ball bearing, and particularly to such a rolling bearing employing a grease having conductivity as a lubricant.
The invention yet further relates to a rolling bearing under a conductive condition between the outer race and the inner race, and more particularly to such a rolling bearing to be favorably used to parts at high temperatures in office machinery or information equipment such as copiers, laser beam printers and others (sensitive drum (fixing part), heat roller supporters).
In general information equipment, for example, in copiers, movable parts use lots of rolling bearings. Between a raceway surface and rolling elements of the rolling bearing, an oil film is formed during rotation to provide a non-contact between both. In the rolling bearing, since static electricity occurs accompanying with the rotation, there probably arises inconveniences that radiation noises thereby give bad influences as distortion and others to copied images of the copier.
For preventing such inconveniences, a prior art has taken a measure that the conductive grease is packed into the rolling bearing to make the inner and outer bearing races and the rolling elements conductive, and one of the inner and outer bearing races is grounded for removing the static electricity from the rolling bearing.
This matter will be explained with reference to FIG. 24 of the attached drawings.
A ball bearing 121 of FIG. 24 comprises an outer race 122, an inner race 123, a plurality balls 124 rotatably arranged between the outer race 122 and the inner race 123, a cage (holder) 125 holding the plural balls 124, and contacting seals 126, 126 furnished in sealing grooves 122b of the outer race 122. A space defined by the outer race 122, the inner race 123, and the seals 126, 126 encircling is charged with the conductive grease 127, and is sealed within the ball bearing 121 by the seals 126.
Contacting faces between raceway surfaces 122a, 123a of both races 122, 123 and balls 124 are lubricated by the conductive grease 127, while the outer race 122, the inner race 123 and the balls 124 are made conductive. Further, the outer race 122 or the inner race 123 is grounded (not shown) through the information equipment, for example, the copiers employing the ball bearing 121 so as to cancel static electricity generated by rotation of the ball bearing 121.
A popular conductive grease is, for example, that carbon black is added as a thickener and a conductive additive (disclosed in JP-B-63-24038), and such a conductive grease displays an excellent conductivity at a beginning period of service. (The term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d)
However, the conventional conductive grease using the carbon black has a problem that it shows the excellent conductivity at the beginning period, but the conductivity goes down as a time passes.
In short, although the rolling bearing packed with the conductive grease shows the excellent conductivity at the beginning period (the inner and outer raceway surfaces and the rolling elements are conductive), the conductivity declines as the time passes so that resistance value between the inner and outer races of the rolling bearing becomes larger (called as xe2x80x9cbearing resistance valuexe2x80x9d hereafter).
Causes for these phenomena have hitherto been considered as follows. The conductive grease enough exists at the beginning in the contacting faces between the rolling elements and the raceway surfaces of the bearing races of the rolling bearing, and the carbon black in the conductive grease maintains the conductivity between the raceway surfaces and the rolling elements, but owing to relative movements between the raceway surfaces and the rolling elements, the conductive grease is expelled from the contacting face, otherwise chain structures of the carbon black particles are broken. Thus, there occurs a problem that the conductivity declines and the bearing resistance value becomes large as the time passes.
Further, it has been assumed that the conductive grease expelled from the contacting face is difficult to again enter the contacting face, because a worked penetration of this kind of greases is low, and the conductive additive is a fine particle insoluble in the base oil.
It has also been assumed that the conductive grease expelled from the contacting face is difficult to again enter the contacting face, because the conductive additive is a fine particle insoluble in the base oil.
It is disclosed in JP-A-1-307516 that a measure for preventing the time-passing decline in the conductivity of the grease is to limit the worked penetration and soften the conductive grease for avoiding the grease from hardening. (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) The conductive fine powder as the carbon black is, if being sole, shorter in capacity as the thickener than generally known thickener as metallic soaps or urea compound, and oil separation degree of the resulting grease exceeds 2%.
However, inventors made earnest studies and have come to an assumption that the time-passing changes of the conductivity (resistance value) are generated by factors as mentioned below.
The rolling bearing (the ball bearing having the inner diameter: 8 mm, the outer diameter: 22 mm, the width: 7 mm) packed with the conventional conductive grease containing the carbon black was offered to a rotation test (radial load (Fr) 19.6N, rotational speed: 150 rpm (minxe2x88x921), rotating time: 500 hours, test temperature: 25xc2x0 C.), and the raceway surface of the rolling bearing after the rotation test was investigated by a scanning type electron microscope (SEM) and an energy dispersive spectrometer (EDS). As examples, FIG. 13 shows an SEM image of the raceway surface of the inner race, and FIG. 14 shows an EDS measuring chart.
From the SEM image of FIG. 13, it is seen that a ground face in the raceway surface of the inner race disappears at a beginning period and wear appear, from which the abrasion is recognized in the raceway surface. Peaks of oxygen are seen in the EDS measuring chart of FIG. 14, from which an oxide film is recognized in the raceway surface.
From them, it may be assumed that the cause for the time-passing change in the conductivity depends on formation of the oxide film in the raceway surface owing to shortage in the lubricating capacity of the conductive grease rather than deterioration in performance of the conductive grease.
That is, fine injuries appear in the raceway surface by metal contact between the surface of the rolling elements and the raceway surface. In the injured part, a new face is exposed, and since the new face has a high activity, it is instantly oxidized by oxygen in an air to form the oxide film. This oxide film deteriorates the conductivity to result in heightening the time-passing resistance value.
From the peaks of the EDS measuring chart, it is seen that components of fine amounts contained in the carbon black form films on the raceway surface by the influence of rotation of bearing. The film of the fine amount component decline the conductivity similarly to the oxide film, and consequently, the time-passing resistance value goes up.
Such phenomenon is a problem common to conductive greases serving between relatively acting members.
As a measure against the phenomenon, it may be considered to use a base oil of high viscosity for securing an oil film and prevent the metal contact, but thickening of the oil film undesirably brings about lowering of the conductive capacity of the conductive grease.
The invention is to solve the problems of the conventional conductive grease as mentioned above, and accordingly it is an object of the invention to provide a conductive grease excellent in conductivity and less to cause a time-passing reduction in the conductivity.
Heat roller supporters or fixing parts of office machinery such as copiers, laser beam printer and others often become high temperatures as about 200xc2x0 C. Therefore, the conductive grease to be used to the rolling bearing for such parts has been difficult to secure the enough conductivity over a long period of service, because a conductive grease using an ordinary lubricant as the base oil is insufficient in heat resistance.
Ordinarily, as the lubricant used as the base oil of the conductive grease, for example, mineral oil, poly-xcex1-olefin oil, ether oil, or ester oil may be taken up, but applicably limiting temperatures of these base oils are 160xc2x0 C. at the most.
Accordingly, rolling bearings to be served to such parts becoming high temperatures as mentioned above have still used a conductive brush to remove static electricity.
The invention is to solve the problems involved with the prior art, and it is an object to provide a rolling bearing displaying excellent in conductivity at from room temperature to high temperatures.
(1) A conductive grease comprising a base oil, a thickener, a conductive solid powder and at least one kind of a wear inhibitor, an extreme pressure agent and an oiliness agent, wherein an amount of the conductive solid powder added is 0.1 to 10 wt % based on a total weight of the grease, and a total amount of the at least one kind of a wear inhibitor, an extreme pressure agent and an oiliness agent added is 0.1 to 10 wt % based on a total weight of the grease.
(2) The conductive grease as set forth in Item (1), wherein the conductive solid powder is at least one kind of particle of which main component is fibrous carbon, metallic particle, metallic compound particle and carbon nano-tube.
(3) The conductive grease as set forth in Item (2), wherein the particle of which main component is fibrous carbon is carbon black or acetylene black.
(4) The conductive grease as set forth in Item (3), wherein the thickener is a metallic soap or a urea compound, each addition amount thereof is 5 to 20 wt % based on a total weight of the grease, and a total amount of the thickener and the conductive solid powder is 5.1 to 20.1 wt % based on the total weight of the grease.
(5) The conductive grease as set forth in Item (1), wherein the wear inhibitor is at least one kind of orthophosphoric acid ester and phosphorous acid ester.
(6) The conductive grease as set forth in Item (1), wherein the extreme pressure agent is at least one kind of DTP (dithiophosphate) metallic compound or DTC (dithiocarbamate) metallic compound.
(7) The conductive grease as set forth in Item (1), wherein the oiliness agent is at least one kind of succinic acid ester, carboxylic acid anhydride, and alkenyl succinic acid anhydride.
(8) The conductive grease as set forth in Item (1) which further comprises fine particle of inorganic compound having an average diameter of 0.05 to 2 xcexcm in an amount of 0.05 to 7 wt % based on a total weight of the grease.
(9) The conductive grease as set forth in Item (1), wherein the base oil is a single or a mixture of at least two kinds of mineral oil, synthetic hydrocarbon oil, ester oil, fluorine oil, ether oil and polyglycol oil, and kinematic viscosity thereof is 5 mm2/s to 120 xcexcm2/s at 40xc2x0 C.
(10) The conductive grease as set forth in Item (1), wherein the amount of the base oil added is 75 to 90 wt % based on the total weight of the grease.
(11) The conductive grease as set forth in Item (1), wherein the oil separation degree at 100xc2x0 C. after 24 hours of the conductive grease is 0.5 to 2 wt %.
(12) The conductive grease as set forth in Item (11), wherein the amount of the conductive solid powder added is 0.2 to 0.9 wt % based on the total weight of the thickener and the conductive solid powder.
(13) The conductive grease as set forth in Item (1), wherein the thickener comprises a main component being at least one of silicone compound and fluorine compound, and the grease comprises carbon black as the conductive solid powder in an amount of 0.2 to 10 wt % based on the total weight of the grease.
(14) The conductive grease as set forth in Item (13), wherein the fluorine compound is polytetrafluoroethylene or ethylene trifluoride.
(15) The conductive grease as set forth in Item (13), wherein the silicone compound is fine powder silica, synthetic mica, mica, or smectite.
(16) The conductive grease as set forth in Item (13) which further comprises at least any one kind of the additives selected from the group consisting of nitrite, benzotriazol, MgO, Ca sulfonate, fluorophosphazen derivative, and MgS2.
(17) The conductive grease as set forth in Item (13), wherein a specific surface area of carbon black is 250 m2/g or more.
(18) The conductive grease as set forth in Item (13), wherein a DBP (dibutyl phthalate) oil supply amount of carbon black is 180 ml/100 g or more.
(19) The conductive grease as set forth in Item (13), wherein the base oil is a straight chain or side chain perfluoroalkylpolyether, perfluoroalkylpolyether introducing a carboxyl or isocyanate group into terminal group thereof, ester-modified or alcohol-modified perfluoroalkylpolyether, or a single or a mixture of at least two kinds of fluorine oil, silicone oil and fluorosilicone oil, and kinematic viscosity thereof is 15 xcexcm2/s to 500 mm2/s at 40xc2x0 C.
(20) The conductive grease as set forth in Item (2), wherein the conductive solid powder is carbon nano-tube having diameter of 1 to 24 nm and length of 0.5 to 30 xcexcm.
(21) The conductive grease as set forth in Item (20) which further comprises, as conductive additive, particles having carbon of carbon black or acetylene lack being main components, metallic particles of gold, silver, copper, tin, zinc or aluminum, or metallic compound particles of silver oxide, niobium sulfide or silver nitrate.
(22) A rolling apparatus comprising an outer member, an inner member and a plurality of rolling elements rotatably arranged between the outer and inner members, and being packed with a conductive grease applied between the inner and outer members and the rolling elements, said conductive grease comprising a base oil, a thickener, a conductive solid powder, and at least one kind of a wear inhibitor, an extreme pressure agent and an oiliness agent, wherein an amount of the conductive solid powder added is 0.1 to 10 wt % based on the total weight of the grease, and a total amount of the at least one kind of a wear inhibitor, an extreme pressure agent and an oiliness agent added is contained 0.1 to 10 wt % based on the total weight of the grease.
(23) The rolling apparatus as set forth in Item (22) which further comprises a contacting seal between the outer member and the inner member, said seal being conductive.
(24) The rolling apparatus as set forth in Item (22), wherein the rolling apparatus is a rolling bearing, the outer member is an outer race, and the inner member is an inner race.
(25) The rolling apparatus as set forth in Item (22), wherein the rolling apparatus is a linear guide, the outer member is a slider, and the inner member is a guide rail.
(26) The rolling apparatus as set forth in Item (22), wherein the rolling apparatus is a ball screw, the outer member is a ball nut, and the inner member is a screw shaft.